Overspill passengers often end up sitting in part of J.
And I have, on several occasions, seen people in that predicament being refused the first class food/drink service. Unbelievable stuff when you consider how much Avanti now charge for 1st - no more bargain Advances like before Covid!
Whichever genius thought 18 seats would be sufficient first class capacity on some of the most important intercity routes in the country needs their head examining.
Avanti also refuse to provide proper compensation on the countless occasions that staff shortages or logistics failures result in no food/drink service. I have frankly given up complaining because I know they will try to fob me off with a 25% off discount code that can't even be combined with a Railcard.
I now only bother using Avanti first class where it's included in my ticket anyway, e.g. on an Interrail pass.